There's much to be said about the size/strength relationship. Several in my dojo are small and don't have the most incredible arm muscles (I know it's not about muscles) so when we're practicing a technique, it's often easy for me (and others) to just stand there while they crank and get nowhere.

Size, strength, flexibility.. all of that matters. But it all matters situationally. Aikido involves choices, and the more we learn the more choices we have. If you don't have much arm strength and can't seem to make sankyo work, try something that doesn't require arm strength, like iriminage. If you're small like one girl in my dojo and you have to hop in order to perform a shomenuchi ikkyo against a tall person, perhaps ikkyo isn't for you, try something low like kotegaishi (tall people hate that one). If you're a big guy going up against a little person, you'll be hard pressed to get low enough to get under their arm for sankyo, so try something that will really knock them off balance: a feint atemi going into any number of throws (atemi from large burly men is quite a distraction). One can say, therefore, that size does matter in that the way you do things will be different and specialized to your size. However, one can also say that size does not matter, as no matter what your size, with training you will be able to perform efficient aikido.

Adults are just outdated children, and the hell with them. - Dr. Suess